Unless it's a steep hill or just joining a road, lorries are fairly good at maintaining a speed. What doesn't help is cars, capable of matching that speed at the very least, are driven much slower meaning lorry drivers wanting to overtake. There is no real excuse for people travelling at 40mph or less on a 70mph dual carriageway or motorway, they cause congestion at the very least, if they aren't confident enough to drive at faster speeds they really ought not be using those roads. These people invariably expect to join fast roads at 40mph as well, barging out in front of faster traffic and not increasing their speed there after. Lorries can be seen from a greater distance, we are all aware they are limited to a maximum speed, where as cars are smaller and you would expect them to join at an adequate speed and continue at an adequate speed there after. From personal experience, people who are driving too slow are usually in their own little world and oblivious to the other cars around them and just as likely to perform dangerous manoeuvres at any time.
